For "point-of-sale" retail transactions, such as when you are paying for groceries at a cash register, Visa and MasterCard prohibit credit card surcharges. Additionally, in some states, like California, it is illegal for a private company to impose a credit card surcharge.
Some exceptions are permitted, however, such as when you use a credit card for payment to a public agency (for things like taxes, certain utilities, a municipal government bill or school tuition).
To report merchants who violate their card agreements by surcharging, use one of these links, depending on whether your card is MasterCard or Visa:
Visa
MasterCard
For American Express or Discover cards, visit the company's website or call the number on the back of your card.